This is a 30-month longitudinal study of 74 severely obese (BMI greater than 40 kg/m2) patients who have undergone gastric bypass (GBP) for weight reduction. It will focus on longitudinal changes in eating behavior, energy metabolism, health status, psychiatric status and psychosocial functioning in 37 binge-eating and 37 non-binge-eating, severely obese subjects followed for 30 months after GBP. It will clarify the outcome of GBP and its prognostic indicators, particularly the role of binge eating and energy expenditure in mediating its outcome. Severe obesity increases both morbidity and mortality and is often resistant to conventional weight loss treatments. Bariatric surgery, particularly GBP, is effective for severe obesity. However, there is great variation in weight loss after GBP and weight regain is common after 18 to 30 months [following] surgery. Furthermore, GBP may not improve binge eating behavior which is very common in severe obesity and may be related to weight regain after GBP. Therefore, the role of binge eating in mediating the outcome of GBP should be studied. Mechanisms that govern energy regulation may influence both eating behavior (including binge eating) as well as energy expenditure. The role of energy expenditure in mediating the outcome of GBP has never been studied. Because of the major weight loss which occurs after GBP, and because weight regain occurs in many patients 18 to 30 months after surgery, the opportunity exists in GBP subjects for a definitive study of energy expenditure in severe obesity. Outcome in psychiatric status and psychosocial functioning after GBP remains unclear. Despite improvement in health status and successful weight loss, suicide is a major cause of death after bariatric surgery. Furthermore, initial improvement in psychosocial functioning may not be sustained despite successful weight loss. Therefore, research in outcome of psychiatric status and psychosocial functioning after GBP is needed. This study is significant because a better understanding of the outcome and the prognostic indicators of GBP may allow better patient selection and the implementation of adjunctive treatments such as cognitive therapy or anti-obesity medications for individuals demonstrating poor outcome predictors or showing persistent psychosocial impairment. Furthermore, it may clarify the role of binge eating and energy expenditure in the pathogenesis of severe obesity.